Aiosmtpd

Despite the less-than-minor version bump, under the hood there are some very notable changes.

Some highlights:

* Simplified `MANIFEST.in` -- this should result in a much smaller package
* Stop using Appveyor and Travis CI, fully using GitHub Actions now for testing
* Lots of improvements in documentation
* Now uses `pytest` for the test runner instead of `nose2`
* Lots of improvements in `tox.ini`
* Some behavior fixing:
* Catch exceptions during `controller.start()`
* More correct SMTP Status Codes
* Correct handling of long lines and/or oversized DATA
* Correct handling of user-supplied TLS Context
* Now implements optional command call limit

This release introduces a Major Feature: **SMTP AUTH with extension support.** Two AUTH mechanisms are internally implemented: `LOGIN` and `PLAIN`, and the 'handler' mechanism has been adapted to cater for additional AUTH mechanisms, if needed. More details available in [the 'Handlers' documentation page](https://github.com/aio-libs/aiosmtpd/blob/master/aiosmtpd/docs/handlers.rst).

In addition, starting 1.2.2 **Python 3.5 support has been dropped**; minimum required Python version is 3.6 (or its analogue with other implementations such as PyPy).

Other changes:
* BUGFIX: Don't strip last `\r\n` prior to terminating dot
* LICENSE: Actually include the license text file for Apache License v2.0
* COMPATIBILITY: Verified compatibility with the latest versions of Python 3.8 and 3.9

This release introduces several bugfixes:

* Improve the documentation on enabling ``STARTTLS``. (Closes 125)
* Add customizable ident field to SMTP class constructor. (Closes 131)
* Remove asyncio.coroutine decorator as it was introduced in Python 3.5.
* Add Controller docstring, explain dual-stack binding. (Closes 140)
* Gracefully handle ASCII decoding exceptions. (Closes 142)
* Fix typo.
* Improve Controller ssl_context documentation.
* Add timeout feature. (Partial fix for 145)